First Vertical Derivative Research Articles

The Aerogeophysical Atlas of Tocantins State: Synthesis of Magnetic Anomaly And Radioelement Maps With Emphasis on Regional Signatures and the Serra Da Cangalha Impact Structure

The Tocantins State Aerogeophysical Atlas (TSAA) is an institutional product of the Geological Survey of Brazil. It is aimed at promoting the use of airborne magnetic and gamma-ray spectrometry data to support geophysical and geological research at state level. Here, we present a synthesis of the TSAA with special emphasis on the interpretation of the magnetic (Anomalous Magnetic Field, First Vertical Derivative (FVD), and Total Gradient Amplitude (TGA) images) and radiometric (RGB composite color map of concentrations, Inverse Images of K, eTh, and eU, and Normalized eU enhancement) map. In particular, the radiometric data for the Serra da Cangalha impact structure in northeast Tocantins are discussed. For Tocantins State, the results show that the radiometric and/or magnetic signatures of geological units (e.g., Mosquito Formation basalts; granite-gneiss/migmatite terrains) and structural features (e.g., dykes and lineaments), or related exogenous deposits (Serra Geral do Tocantins quartzose soils; alluvial deposits of main drainage channels) can be visualized well on regional-scale maps. Analysis of radiometric ratios for the area of the Serra da Cangalha impact structure indicates that eU concentrations are enriched in the annular basin around the central uplift structure, and in the zone straddling the contact between the Piauí and Pedra de Fogo formations. In the latter case, the elevated eU concentrations could be related to brecciated chert layers or silicified sandstones.

Integrated Geophysical Investigation for Potential Gold Mineralised Zone within Lower Part of Zuru Schist Belts, NW Nigeria

The study began with a reconnaissance survey that used high-resolution aeromagnetic data of the study area to identify suitable structure zones with the potential to trap mineral resources within the context of the gold exploration target. The findings of magnetic studies were followed up with detailed investigations using 2D electrical resistivity tomography (ERT) and induced polorisation (IP) techniques to reveal further details about gold mineral zones and other lithological boundaries. The airborne magnetic data of sheet 118_Yelwa was obtained from the NGSA; these datasets were processed and analyzed using Oasis Montaj's first vertical derivative (FVD) and center for exploration targeting (CET) techniques. Results of FVD and CET grid anomalies show that regions of major magnetic structures (lineaments) are associated with granite gneiss, migmatitic augen gneiss, and medium to coarse-grained biotite when compared to the geological settings of the area. The zones of major structures obtained in this study coincided with previous magnetic studies of the area, located in the eastern parts of Ngaski, Yauri (Yelwa), Shanga, Agwara, as well as Magama's northwest region. Some of the regions for lineament (in the eastern part of Ngaski/Yauri) were investigated further with 2D ERT and IP detailed geophysical methods in a dipole-dipole configuration. The results of geoelectric techniques along profiles 1, 2, and 3 identified the major gold mineralisation potential zones, which were labeled A1, A2, and A3. These regions have low/high resistivity (1.6 to 459 Ωm/1889 to 7773 Ωm) and chargeability signatures (≥ 20 msec), and could thus be interpreted as potential target zones for metallic mineral exploration, particularly gold mineralisation. The regions are located in the northern Mararraba and southwest of the Jinsani areas of Kebbi State. The results of integrated geophysical methods have produced updated structural features of the regions, as well as a database containing precise locations, lateral lengths,...

Integrated Geophysical Investigation for Potential Gold Mineralised Zone within Lower Part of Zuru Schist Belts, NW Nigeria

The study began with a reconnaissance survey that used high-resolution aeromagnetic data of the study area to identify suitable structure zones with the potential to trap mineral resources within the context of the gold exploration target. The findings of magnetic studies were followed up with detailed investigations using 2D electrical resistivity tomography (ERT) and induced polorisation (IP) techniques to reveal further details about gold mineral zones and other lithological boundaries. The airborne magnetic data of sheet 118_Yelwa was obtained from the NGSA; these datasets were processed and analyzed using Oasis Montaj's first vertical derivative (FVD) and center for exploration targeting (CET) techniques. Results of FVD and CET grid anomalies show that regions of major magnetic structures (lineaments) are associated with granite gneiss, migmatitic augen gneiss, and medium to coarse-grained biotite when compared to the geological settings of the area. The zones of major structures obtained in this study coincided with previous magnetic studies of the area, located in the eastern parts of Ngaski, Yauri (Yelwa), Shanga, Agwara, as well as Magama's northwest region. Some of the regions for lineament (in the eastern part of Ngaski/Yauri) were investigated further with 2D ERT and IP detailed geophysical methods in a dipole-dipole configuration. The results of geoelectric techniques along profiles 1, 2, and 3 identified the major gold mineralisation potential zones, which were labeled A1, A2, and A3. These regions have low/high resistivity (1.6 to 459 Ωm/1889 to 7773 Ωm) and chargeability signatures (≥ 20 msec), and could thus be interpreted as potential target zones for metallic mineral exploration, particularly gold mineralisation. The regions are located in the northern Mararraba and southwest of the Jinsani areas of Kebbi State. The results of integrated geophysical methods have produced updated structural features of the regions, as well as a database containing precise locations, lateral lengths,...

GIS-based multi-criteria decision models for barite exploration in Nigeria’s Benue Trough

Spatial predictive mapping using geographic information system (GIS) is considered an invaluable tool for reconnaissance-scale exploration of mineral resources. In this study, geospatial data on geophysics, remote sensing, and structural and lithological attributes were systematically integrated to prospect barite potential zones within the Mid-Nigerian Benue Trough (MBT). Correlation attribute evaluation was used to establish the relationship between mineral deposit occurrences and geospatial data, while data integration was implemented using the Multi-Objective Optimization by Ratio Analysis (MOORA), Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS), and Additive Ratio Assessment (ARAS) multi-criteria models. Here we show that the correlation attribute evaluation suggests that barite occurrences displayed a strong correlation with spatial data on lineament density, ferric iron alteration, and potassium to thorium (K/Th) ratio, whereas a weak correlation was observed with spatial data on the first vertical derivative (FVD), proximity to the host rock, and ferrous iron alteration. Here we report that the quantitative estimation of predictive models indicated that very high predictive zones for barite occurrences accounted for 19% of all the models. The accuracy assessment using Receiver Operating Characteristic (ROC)/Area Under the Curve (AUC) showed prediction levels above 78% for all models. The effectiveness of the spatial application of multi-criteria decision models makes them a reliable tool for barite exploration within the Mid-Nigerian Benue Trough (MBT) and other geologically similar environments.

Application of Airborne Magnetic Data to Assess the Structures that Favour Gold Mineralization within Southern Parts of Katsina State, Nigeria

The Nigerian schist belts are endowed with gold mineralisation and to delineate the geological structures that favour gold mineralisation, the airborne magnetic data that cover the study area was employed. Also, the residual magnetic anomaly data of the research area was reduced to the Equator (RTE) followed by the application of the centre for exploration targeting (CET) plug-in and three source edge mapping techniques; analytic signal (AS), first vertical derivative (FVD) and the horizontal gradient magnitude (HGM), for structural delineation and subsequently the structural density map of the area was produced. Structural density varies from 0.186 km/km2 low to 0.93 km/km2 high. Portions within the study area with high structural densities are Bakori in the southwest, Shanono in the northeast, Gwarzo situated directly south of Shanono, north and south of Karaye, the border between Kafur and Rogo in the south, all within the schist. Since structures are conduits for the migration of mineralisation fluids, areas with high structural densities will be viable for gold mineralisation. Also, comparing the structural density map of the area to the analytic signal map of the area has revealed the high structural density portions to coincide with the high analytic signal zones. This implies that structures within these portions are responsible for the migration of mineralization fluid into these environments.

Integration of airborne magnetic and geological data for mineral prospectivity mapping in the Ambam-Amvom rainforest area, Archean Ntem Complex, southern Cameroon

The accurate mapping of mineral resources in rainforest areas is challenging due to dense vegetation and thick lateritic cover obscuring rock outcrops. This study utilized airborne magnetic survey data to map geological features and target mineral resources in the Ambam-Amvom area at the NW margin of the Congo craton. Linear and non-linear filtering algorithms were used, including reduction to the equator (RTE), upward continuation (UC) at 1000 m and 2000 m, first vertical derivative (FVD), and analytic signal (AS). Field surveys were carried out for sampling and petrographic investigations. The TMI map showed magnetic signals varying from -4489 nT to 2317 nT, while the RTE map displayed magnetic signals varying from -2984 nT to 3528 nT. The RTE-Reg map exhibited anomaly signal variations ranging between -2704 nT and +3637 nT and between -2829 and +3864 nT, respectively for UC at 1000 m and 2000 m. Geological structures were interpreted from the FVD, AS, and TMI-RTE images, and linear structures of regional extend mainly trend NW-SE, E-W, and NE-SW. Field investigations revealed that the study area comprises iron formations, amphibolite, gneiss, gabbro, biotite granite, and charnockite. The bulk chemical composition of iron formations showed a high content of TFe, averaging 39.80 wt.% for itabirite, 53.79 wt.% for hematite-geothite ore and 59.63 wt.% for quartz-hematite ore. The zones of peak magnetic anomalies coincided with the occurrence of the greenstone belt, and the integration of aeromagnetic and geological data led to the identification of three main target zones for iron ore minimization. High fracture density zones within the research were identified as high prospective targets for gold mineralization. The study's findings highlight the high potential of the Ntem Complex for mineral resources and provide valuable information for future research and development.

The Studying of Magnetic Anomalies Due to Shallow Underground Targets and the Environmental Applications.

The magnetic method of investigation is a well-known and dependable technique for detecting underground ferro-materials. However, the application of the magnetic method to identify the exact position of underground targets such as buried explosive devices (BED), illegally buried waste (IBW), metallic storage tanks, utility pipes, and communication cables is quite challenging in both civil and environmental engineering. This article presents the results of the investigation performed at a 55m by 55m area within the Ahmadu Bello University Geophysical Test Site (ABUGTS) to study the signals generated by similar buried targets to simulate situations encountered in environmental engineering. The total magnetic intensity (TMI) of the site without buried targets was obtained to identify the possible existing underground materials or faults that could generate false geophysical data. The magnetic susceptibility of the buried targets was determined in the laboratory before the burial to validate the field results. The influence of the buried materials over the magnetic signature of the site was examined, showing the TMI values vary between 34966 nT and 44510 nT before burial and 351607 nT to 49795 nT after burial. The Euler deconvolution (ED) and residual maps alongside the signal enhancement maps like analytical signal and first vertical derivative (FVD) were employed to study the magnetic anomalies due to buried targets. The results of the data interpretation confirmed the existence of cluster anomaly source bodies due to buried targets. The ED delineates the depth of the buried targets varying between 0.7m – 3.3m corresponding with the actual depth of burial. The analytical signal map produced a perfect behaviour of the magnetic anomalies to buried targets, capturing all the buried targets buried at the site. However, the shape of the anomaly signal is almost independent of the buried target orientation with a spherical pattern of anomalies. However, the exact size and shape of the buried targets were not achieved. Nevertheless, the FVD and analytical signal maps perform better than the residual and TMI maps due to the substantially smaller magnetic anomalies of the buried targets similar to expected. In addition, stronger magnetic anomaly signals occur maximally around buried larger ferro-materials with precise prediction of the buried target position. The magnetic method is a very powerful, convenient, cheap, and time-saving geophysical tool for investigating underground targets, which is highly applicable in engineering and environmental contexts.

Mapping of geological structures and sediment thickness from analysis of aeromagnetic data over the Obudu Basement Complex of Nigeria

Abstract In this study, geologic structures, as well as attendant orientations and sediment thickness, in the Nigerian Obudu Complex were delineated using the Centre for Exploration Targeting (CET), and depth determination methods such as source parameter imaging (SPI) and standard Euler deconvolution (SED). The CET, SPI, and SED procedures were applied on the total magnetic intensity data. Also, the enhanced TMI data using analytic signal, first-vertical derivative, total-horizontal derivative, and tilt-angle derivative filters were further subjected to CET operation, with the aim of mapping both subtle and prominent lineaments. In general, mapped geologic structures trends in the NE–SW, NNE–SSW, E–W, and N–S directions. Overall, the dominant geologic structural orientations of NE–SW and NNE–SSW reflect the regional strike orientation. The regional striking of the lineation, which is caused by the Pan-African orogeny and subsequent post-orogenic processes, has an impact on these orientations. The N–S and E–W structural deviations from the main NE–SW and NNE–SSW trends are initiated by the YGS of the post-orogenic events. Overall, these complex geologic structures are probable sites for metallogenic minerals.

Application of enhanced methods of gravity data analysis for mapping the subsurface structure of the bahira basin in Morocco

Gravity-based imaging of the subsurface has increased worldwide recently. Improvements in the processing and analysis of gravity data have allowed us to locate the basement surface, map geologic basins, and define structural patterns. In this study, gravity data were analyzed to study the Bahira basin’s underlying geology. The Bahira basin is very important economically. The Ganntour plateau is distinctive due to the importance of the phosphate mining resources. Using gravity data, we mapped the subsurface and determined the underlying structural patterns that affect the study area. In this study, we used several techniques to edge detection including Total horizontal derivative (THDR), first vertical derivative (FVD), tilt derivative (TDR), and its horizontal derivative (THDR_TDR) methods. Accordingly, the geological history of the Bahira basin suggests that the main lineaments/faults trends are NE-SW, NW-SE, ENE-WSW, and WNW-ESE. The 3D Euler deconvolution showed the depth and location of lineaments/faults, and matched edge detection results. The eastern Bahira basin’s sedimentary layer is 2–8 km deeper according to the Euler technique. Two-dimensional forward modeling along three profiles in the Bahira basin revealed a horst-graben basement structure. The outcomes of this study improved the subsurface topographical variations of the Bahira Basin. The information collected so far can help future studies in the area.

Application of Microgravity Survey in the Exploration of Rare-metal Bearing Pegmatites – A Case Study from Marlagalla, Mandya District, Karnataka

Abstract Geologically, the Marlagalla area represents the southern part of NNE-SSW trending Nagamangala schist belt, which is a member of ancient supracrustals of Sargur Group and situated in the southern part of Western Dharwar Craton. The area is mostly soil cover with limited exposure of amphibolite/hornblende schist and intrusive pegmatite. Pegmatites occur as lensoidal bodies within the amphibolite.Microgravity surveys are attempted for the first time to detect pegmatites within the amphibolite. The density between pegmatite (Density= 2.58-2.61 gm/cc) and host rock amphibolite (Density=3.09-3.14 gm/cc) gives detectable contrast to identify pegmatite within amphibolite. Gravity data are acquired over a grid of 5m x 30m using CG-5 Autograv gravimeter which has 1 μGal sensitivity. Elevation data are acquired using SP80 post processing DGPS instrument which has 5 mm elevation accuracy. Analysis of residual gravity and first vertical derivative (FVD) filters facilitated in delineating the pegmatite bodies. The pegmatite bodies show low gravity anomalies with an amplitude range of -5 to -200 μGal. FVD map of the residual gravity indicated four major N100E trending linear anomalies of -36 to 3.0 μGal/m corresponding to pegmatite bodies of varying strike (100-200m) and width (10-15m).2D-Forward modeling of microgravity data helped in deriving the subsurface geometry of the pegmatites. Borehole drilled subsequently based on the results of microgravity surveys intercepted pegmatites within the amphibolite. Thus, the technique has proved for exploration of pegmatite in contrasting environment of amphibolite. Pegmatite are known for rare metals and lithium minerals. Indirectly the model proposed here will help in exploration of rare metals and Li bearing pegmatite.

Aeromagnetic Mapping and Radioelement Influence on Mineralogical Composition of Mesothermal Gold Deposit in Part of Ilesha Schist Belt, Southwestern Nigeria

ABSTRACT Airborne magnetic and radiometry data sets over potential gold mineralisation associated with mesothermal alteration zones in the western Ilesha schist belt, Southwestern Nigeria, were interpreted. This was done to provide information on possible extent of geologic transformations that accompanied gold mineralisation in the area. Interpreted gamma spectrometric data were successfully used in delineating zones of hydrothermal alteration associated with potassium K enrichment as the target for gold deposits. The geological structural features that host the ore deposits were identified as lineament represented by lithological contacts and faults/fractures that were successfully exacted from the airborne magnetic data. The potassium deviation (KD) map was computed to enhance the potassium signature of rocks in the area of study. Thus, the KD map represents real potassium distribution across the study area emanated from hydrothermal alteration where hydrothermalised zones were displayed by high KD values. First vertical derivative (FVD) and total horizontal gradient (THG) maps were used to delineate lineaments. These lineaments were connected using the frequency rose diagram with two main lineament set,; major and minor lineaments observed. The 3D Euler deconvolution (EUD) method was also applied on the THG map to locate and evaluate depths to subsurface structures. The best 3D EUD solution for dykes and contacts modelled using the Structural Index of zero (S.I = 0) was used to estimate the depth to these anomaly sources at 300 to 700 m in the study area. The EUD results also revealed several subsurface structures which were hidden in the existing geological map of the study area. A prospective mineralisation map was produced from the synthesis of both magnetic lineaments and alteration zones maps derived for the study area, showing the areas of probable high mineral resources. Strong relationships were observed between the mapped hydrothermal altered zoneseologic structures and superimpose known gold mining pits.

Comparative analysis of AVIRIS-NG and Landsat-8 OLI data for lithological mapping in parts of Sittampundi layered complex, Tamil Nadu, India

The present study addresses a comparative analysis of the potentials of Airborne Visible and infrared imaging spectrometer -next generation (AVIRIS-NG) data, with respect to Landsat-8 Operational Land Imager (OLI) data for mapping of sporadic exposures of mafic cumulates within the anorthosite intrusion and delineation of major lithological units of Sittampundi layered complex. AVIRIS-NG and Landsat-8 OLI data were initially compared using scene-based relative signal-to-noise ratio (SNR) analysis. Subsequently, spectral anomaly maps were generated to map different lithological rock units using the constrained energy minimization (CEM) approach. Spectral maps derived using CEM were further filtered by low-pass filtering to control the false-positive result. Spectral angle mapper (SAM) classification technique was applied to analyze the suitability of AVIRIS-NG and Landsat-8 OLI data for lithological mapping. Confusion matrix analysis was carried out to assess the SAM classified maps. The first vertical derivative (FVD) map of magnetic anomaly and field evidence/geological exposures were used to validate the results. SNR analysis indicates the superior signal strength of AVIRIS-NG data than Landsat-8 OLI data. Moreover, combined analysis based on CEM and SAM of remote sensing data and FVD of magnetic anomaly reveals that the false-positive area indicating metagabbro/mafic cumulate is lower in AVIRIS-NG data than Landsat-8 OLI data. This is because of the relatively poor SNR, spectral and spatial resolutions of Landsat-8 OLI data; however, it can be good enough for mapping of metagabbro/mafic cumulate when AVIRIS-NG data is unavailable.

Aeromagnetic and remote sensing characterization of structural elements influencing iron ore deposits and other mineralization in Kabba, southwestern Nigeria

Remote sensing and aeromagnetic data analyses were conducted for the characterization of basement structural elements that influence iron ore deposits and other mineralization over kabba and its surroundings. Spectral analysis suggests depth to magnetic sources range from deep, intermediate, and shallow with depths (i.e., 2.8 km, 1.5 km, and 0.3 km respectively). Three lithological zones were revealed on the Analytic Signal (AS) image which includes Meta-Sedimentary Complex, migmatite-gneiss-quartzite complexes, and undifferentiated schist and metavolcanic rocks. Structures interpreted based on first vertical derivatives (FVD), total horizontal derivatives (THD), and AS of the aeromagnetic images enabled delineation of long, deep, regional, inter-terrain linear structural features that are different from shallow, relatively short local intra-terrain structures. Typical potential mineralized zones are regions where the delineated structures on the AS coincide with FVD anomalies vis-a-vis zones exhibiting THD peak anomalies and high magnetic susceptibility. These characterized zones also coincide with the location of known geologically deposits of iron - ore and marble. Mineral deposits tend to be dominated regionally by the dominant structural patterns (NE-SW to ENE-WSW, NW-SE to WNW-ESE, and E-W) delineated from SRTM and aeromagnetic analysis.

INTERPRETATION OF AEROMAGNETIC DATA AND LANDSAT IMAGERY OVER THE NIGERIAN YOUNGER GRANITES IN AND AROUND KAFANCHAN AREA, NORTH-CENTRAL NIGERIA

In this research the lineaments of the Kafanchan area in North-central Nigeria were investigated in order to explore the mineralization zones of the area. Aeromagnetic data over Kafanchan and environs within the Younger Granite Province, in the North-Central Nigeria were collated and analyzed. The aeromagnetic map of the area was interpreted both qualitatively and quantitatively so as to identify the nature of the magnetic sources and the trends direction in the study area. The trend of the Total Magnetic Intensity (TMI) map is predominantly in NE-SW. The First Vertical Derivative (FVD) Lineaments Map was also correlated with LADSAT lineaments map and both maps agreed in most areas. The study area is characterized by predominant magnetic lineament trend in NE-SW direction and subordinate E-W direction. The result also shows that the most significant structural trends affecting the distribution of these magnetic anomalies in the study area is in NE-SW direction. The TMI map indicates that there are three major mineralization zones in the study area. The high magnetization contrast in the NE and SE parts of the study area correlates with the migmatite-gneiss, biotite-granites, granites and basalts which are associated with high magnetic contrasts. Also, the high magnetization contrast in the NW part of the area correlates with basalt and the biotite-granite. However, the predominant low magnetization contrast observed in the western half does not correlate with the basic igneous rock

Structural interpretation of High-resolution aeromagnetic data over the Dahomey basin, Nigeria: implications for hydrocarbon prospectivity

The renewed quest to boost Nigeria’s dwindling reserves through aggressive search for oil and gas deposits in Cretaceous sedimentary basins has re-ignited the need to re-evaluate the hydrocarbon potentials of the Dahomey Basin. Aeromagnetic data are a low-cost geophysical tool deployed in mapping regional basement structures and determination of basement depths and sedimentary thickness in frontier basin exploration. In this study, high-resolution aeromagnetic (HRAM) data covering the Dahomey Basin Nigeria have been interpreted to map the basement structural configuration and to identify mini-basins favorable for hydrocarbon prospectivity. The total magnetic intensity grid was reduced to the equator (RTE) and edge detection filters including first vertical derivative (FVD), total horizontal derivative (THDR), tilt derivative (TDR) and total horizontal derivative of upward continuation (THDR_UC)) were applied to the RTE grid to locate the edges and contacts of geological structures in the basin. Depth to magnetic sources were estimated using the source parameter imaging (SPI) method. Data interpretation results revealed shallow and deep-seated linear features trending in the NNE-SSW, NE-SW, NW-SE and WNW-ESE directions. The SPI map showed a rugged basement topography which depicted a horst-graben architecture on 2D forward models along some selected profiles. Two mini-basins ranging in basement depths between 4.5 – 6.3km were mapped offshore of the study area. It appears the offshore Dahomey Basin holds greater promises for hydrocarbon occurrence due to the presence of thicker succession of sedimentary deposits in the identified mini-basins.

Digital filters applications on aeromagnetic data for identification of hidden objects

The hidden objects are located, as sixteen objects (anomalies) on the reduction to magnetic pole data (RTP). The follow-up and identification of these objects by applying several digital filters, as upwarding continuation at different level, then the horizontal derivative (HD) and vertical derivative (VD). Accordingly, the edge detection filter techniques are dependent on the tilt derivative (horizontal, vertical derivatives) and the tilt angle. This study shows closing to the basic dykes (shallow anomalies) that imprinted in the RTP map and the upward continuation maps at shallow levels. The residual features are mostly starting to disappear gradually in the upward continuation maps at intermediate and deeper levels. The last upwarding maps illustrate the penetrated roots of these bodies to deep levels (deep anomalies). The advanced filter technique of edge detection is the enhanced total horizontal derivative of the tilt angle (ETHDR). This study delineates and compares the hidden objects by applying the different filter techniques, as the analytical signal (AS), first vertical derivative (FVD), total horizontal derivative (THDR), tilt angle (TA), horizontal tilt angle (TDX), and ETHDR. The applied derivative-based filters, as ETHDR, produce more detailed results for the shallower and deeper magnetized structures. The compared filter results such as TA, TDX, and ETHDR give sharp edges of the magnetized sources (hidden objects). Quantitatively, the determined depths have been evaluated, using the 2D modeling algorithm. The depths and relief of the basement surface across eight profiles are defined, and the depths and shapes of the hidden objects are outlined. Meanwhile, magnetic susceptibility contrasts of the hidden objects are varied with different expanding shapes and subsurface depths.

Aeromagnetic interpretation of basement structure and architecture of the Dahomey Basin, Southwestern Nigeria

ABSTRACT Significant yet untapped resources still abound in the Nigerian sector of the Dahomey Basin. Although the presence of an extensive Cretaceous Petroleum System has been confirmed following recent discoveries in Aje and Ogo Fields offshore Lagos, exploration outputs in the Dahomey Basin has so far not been encouraging. Proper understanding of the basement architectural framework and controls on tectonic development remains key to unlock the unrealised potentials in the basin. Hence, a geophysical interpretation of the basement structure and architecture of the Dahomey Basin southwestern Nigeria has been carried out in this study. Various edge enhancement techniques were applied to the high-resolution residual magnetic intensity (HRRMI) grid of the area. This includes first vertical derivative (FVD), total horizontal derivative (THDR), tilt derivative (TDR) and total horizontal derivative of upward continuation (10 km). Determination of the depth to magnetic sources and sedimentary thicknesses in the study area were achieved using standard Euler deconvolution and source parameter imaging (SPI) techniques, with depth range of 4.5–6.3 km attained in the two identified sub-basins located offshore of the study area. Lineament analysis gave insights on the tectonic trends and stress-field orientation in the basin with major trends in the NNE-SSW, NE-SW, NW-SE, and WNW-ESE directions. 2D forward modelling of some selected profiles was employed to characterise the basement pattern and architecture, which depicted a horst-graben architecture. The basement structure and architecture have a major control on the distribution of sub-basins, petroleum systems elements and trap styles in the basin. The study demonstrates the robust application of high-resolution aeromagnetic data in basin-wide mapping of regional subsurface geological features, basement architecture and determination of sedimentary thickness in a frontier basin.

Integrating of Remote Sensing and Airborne Magnetic Data to Outline the Geologic Structural Lineaments That Controlled Mineralization Deposits for the Area around Gabal El-Niteishat, Central Eastern Desert, Egypt

The Gabal (G.) El-Niteishat area lies in the Central Eastern Desert of Egypt which is known for various mineral resources and geological structures. Umm Gheig, Umm Naggat, Umm Shaddad, Wadi (W.) Zeidun and Sigdit represent some important regions that contain mineral deposits in the study area. Various filters such as first vertical derivative (FVD), horizontal gradient magnitude (HGM), tilt derivative (TDR) and near-surface were applied to the airborne magnetic data for the study area to deduce the structural lineaments and magnetic source edges which were controlled by the presence of mineral deposits. Processed Landsat ETM+ images are used for delineating the rock unit boundaries that are exposed in the study area such as serpentinite, metagabbro, metavolcanics and metasediments. Also, band ratios, principal component analysis (PCA) and false-color composite image (Crosta alteration image) were applied to get specific results about the alteration zones. The structural lineaments analysis illustrated that the common trends that affected the study area were NW-SE, NE-SW, E-W and N-S. Integration of remote sensing and airborne magnetic data exhibited the relation between mineralization and structural lineaments.

INTEGRATION OF MAGNETIC RESIDUALS, DERIVATIVES AND LOCATED EULER DECONVOLUTION FOR STRUCTURAL AND GEOLOGIC MAPPING OF PARTS OF THE PRECAMBRIAN GNEISSES OF AGO-IWOYE, SOUTHWESTERN NIGERIA

Ground based magnetic survey conducted between longitude 06O 55I 51IIN – 06O 55I 54IIN and latitude 03O 52I 06IIE – 03O 52I 4.8IIE (Olabisi Onabanjo University) remarkably revealed a consistent subsurface NW - SE structural azimuth of localized discontinuities within the shallowly buried heterogeneous basement rocks, which at exposed locations are composed of strongly foliated granite gneiss and migmatite-gneiss with veins and veinlets principally orientated in NNW – SSE direction. Magnetic survey of the area was preceded by site inspection to avoid metallic objects interferences. Field procedure in the area involved Cartesian gridding, base station establishment, data acquisition at gridded points, and repeated bihourly diurnal checks at the base station. At the processing stage, diurnal variation effect was aptly removed before subjection to Kriging (gridding). The gridded data was then prepared as input for Forward Fourier Filter Transform (FFT), which upon definition and implementation enabled Butterworth filtering of isolated ringing effects, reduction to the equator (RTE) for geomagnetic correction, and the use of Gaussian and Upward Continuation filtering for regional magnetic intensity trend determination. Removal of the regional magnetic intensity (RMI) from the total magnetic intensity (TMI) resulted in the derivation of the residual anomaly. Enhancement filters adopted for better resolution of the residual magnetic gradient include analytical signal (AS), tilt-angle derivative (TDR), vertical derivative deconvolution (VDD), and the first vertical derivatives (FVD). TMI and RMI values range between 32925nT – 33050nT and 32935nT – 333050nT respectively, while the residual gradient ranges between 15nT/m and 10nT/m; AS ranges between 0.28nT/m and 4.1nT/m; and TDR ranges from -1.4nT/m to 1.4nT/m. Source depth calculation estimated from power spectrum analysis and Euler deconvolution ranges between 1m and 15m. Composite overlay of magnetic maps revealed jointed and faulted zones within the area; exhibiting a NW-SE principal azimuth of Liberian orogenic impress, which are in consistence with the foliation direction of the jagged foliated bedrock with an estimated maximum overburden of about 15m. The structural significance of this area as a prospective hydro-geological centre, and as an undesirable spot for high-rise building has been accurately evaluated from research findings. Application of integrated geophysical approach, complemented by detailed geological studies may furnish greater information about the subsurface structural architecture. Keywords: Gneisses; Ground Magnetic Surveying; RTE; Structural discontinuities; TDR. DOI 10.35180/gse-2020-0027

Edge position detection and depth estimation from gravity data with application to mineral exploration

A method is applied to residual gravity data taken from the Ortaklar mine, Turkey to delineate the edge position and to estimate the depth of a buried ore body. The horizontal gradient magnitude (HGM) and tilt angle map (TAM) techniques are applied to the first vertical derivative (FVD) of the residual gravity data. The maxima contours of the HGM data and the zero contours of the TAM correspond to the edges of the ore body. The half distance between ± 45° (± 0.785 rad) TAM contours is used to determine the depths of the body. The obtained results are then compared against the depths retrieved from the Grav2dc inversion program, which inverted two profiles taken across the maximum response of the gravity anomaly map. The depths of the ore body estimated by the two methods are found in good agreement as reveal from drilling information.